High voltage bushings are used for carrying current at high potential through a plane, often referred to as a grounded plane, where the plane is at a different potential than the current path. High voltage bushings are designed to electrically insulate a high voltage conductor, located inside the bushing, from the grounded plane. The grounded plane can for example be a transformer tank or a wall.
In order to obtain a smoothening of the electrical potential distribution between the conductor and the grounded plane, a bushing often comprises a number of floating, coaxial foils made of a conducting material and coaxially surrounding the high voltage conductor, the coaxial foils forming a so called condenser core. The foils could for example be made of aluminium, and are typically separated by a dielectric insulating material, such as for example oil impregnated paper (OIP) or resin impregnated paper (RIP). The coaxial foils serve to smoothen the electric field distribution between the outside of the bushing and the inner high voltage conductor, thus reducing the local field enhancement. The coaxial foils help to form a more homogeneous electric field, and thereby reduce the risk for electric breakdown and subsequent thermal damage. OIP is used with oil-filled bushings, while RIP is used in dry-type bushings.
An RIP condenser core is produced by winding paper sheets in concentrical layers and positioning aluminium foils between some of the paper sheets such that the foils are insulated from each other. Under vacuum, epoxy resin is impregnated into the dry layers of wound paper, after which the resin is cured to produce the RIP core.
Some RIP condenser cores, are wound directly on the conductor. A potential connection is made between the conductor and the innermost foil in the core in order to achieve an environment within the innermost foil which is free of an electrical field. However, it may practical to be able to exchange the conductor, e.g. chose between a cupper or an aluminium conductor why a condenser core which is produced separate from the conductor and allows the conductor to be introduced through the core may be desired. This can be achieved by winding the core on a mandrel which is then removed to provide a longitudinal through hole in the core through which the conductor can be introduced. However, especially for larger cores, it may be difficult to remove the mandrel after winding due to shrinkage of the core during manufacture, which clamps the core to the mandrel. Another possibility is to wind the condenser core on a metal winding tube, usually of thin aluminium or copper. A reason for using a winding tube of a conducting metal is to be able to easily have a potential connection between the conductor/winding tube and the innermost foil in the condenser core. The winding tube remains in the core and provides the longitudinal through hole through which the conductor is inserted.
In an RIP condenser core with a winding tube, the thermal expansion coefficient of the RIP is in the order of three to five times higher than that of the aluminium or copper of the winding tube. Since the cross section area of the RIP in the core is significantly larger than that of the winding tube, the RIP will govern the thermal expansion of the core. This result in either the metal winding tube being delaminated from the RIP material or in high mechanical tension stresses in the winding tube. The RIP core may be designed such that the core is supposed to stick to the winding tube at one position whilst the rest is supposed to be able to separate from the winding tube during expansion of the RIP (by the use of e.g. cork, rubber and sealing). Occasionally the RIP core sticks to the winding tube anyway, which can destroy the winding tube.